Certain types of food products which are sterile when packaged may ferment once the containers have been opened and sterility has been lost. Such fermentation is especially rapid with fruit juices. Fermentation produces carbon dioxide gas which, if the container closure is sealed, causes gas pressure inside the container to increase, especially if there is relatively little "head space" above the level of the food product in the container. In extreme cases pressure inside the sealed container can reach a level so high as to rupture and shatter a glass container and thus present a possibility of injury. In the packaging industry, internal gas pressures in excess of about 40 psi are considered undesirably high. Excessive internal pressure in a container can also arise if a jar of food is heated in a microwave oven without first removing or at least loosening the closure.
Pressure venting closures which release internal pressures in excess of a preestablished limit are known. One widely used type of pressure venting closure is described in Lecinski U.S. Pat. No. 3,690,497. That closure is a single piece metal closure having a molded gasket around the inside edge of the top and extending downwardly on the skirt of the closure. Threads are formed in the moldable gasket material while it is held in place on the container. If internal pressure becomes excessive, the threads are stripped and the closure pops off. However, undesirably high torque is required to unscrew the closure and, once the closure has been removed, resealing it is sometimes difficult.
Composite pressure venting closures are also known, having an outer shell and an insert disk or top cover which is received in the top opening of the shell, the gasket being provided on the lower face of the insert disk. Pressure is vented by upward movement of the disk within the shell, away from the container finish. My above-identified co-pending application Ser. No. 401,999 discloses one such composite, pressure venting closure. In that closure the shell has a top lip which overhangs the edge of the insert disk, above the container. The top lip of the shell has one or more recesses areas on its undersurface, which permit excessive gas pressure to deflect the disk upwardly and locally away from the rim of the container, toward and into the recessed areas, sufficiently to release excess gas pressure past the seal. Bosses or bearing areas on the undersurface of the top lip between the recessed areas prevent the recessed areas from being blocked by over-tightening the closure.
The closure shown in my co-pending application is a so-called "top seal" closure because the seal is substantially formed by and between a gasket carried on the insert disk and the top or rim of the finish. Top seal closures are useful in packaging most food products which can generate internal pressure requiring venting.
Because the gasket of a top seal closure engages only a relatively narrow area on the container rim, the sealing area may not be wide enough to seal food products which are especially sensitive to ingress of oxygen prior to opening, and which thus require a better seal. Certain types of processed food products (for example, meat products such as veal baby food) are sensitive to much lower levels of oxygen leakage into the container than other kinds of food products. (Because most of the common highly oxygen sensitive processed food products are meat products, the term "meat products" as used herein is meant to refer to and include all the highly oxygen sensitive food products, exemplified by meats, as distinguished from less oxygen sensitive food products.)
Top seal closures permit a small but undesirable degree of migration of oxygen (air) through the region of contact of the gasket with the rim of the glass container, into the partially evacuated area over the food product. Because of this leakage, a "better" seal, i.e., one which presents a longer distance through which the oxygen must migrate to enter the evacuated space, and/or which has a gasket with lower oxygen permeability, is needed for packaging meat products.
The need for a better seal for meat products generally requires both a container with a special seal area configuration and a special closure for use with the special container. Typically such seals are significantly "longer" and also are less oxygen permeable than seals for non-meat food products. Since the width of the rim of a container is limited by practical considerations, effective seal "length" is increased by extending the seal area downwardly from the top seal area, i.e., by increasing the effective width of the seal area. The region of contact between the closure gasket and the container includes a slanting or vertical side seal portion of the container finish, as well as the generally horizontal or upwardly facing rim of the container. The effective length of such a sealing area, i.e., the seal dimension across the top and down the side, may be two or three times that of a top seal alone. The resulting seal, including both the seal area at the top and the area on the side, is referred to as a "side seal."
Apart from differences in configuration between side seals and top seals, the gasket materials usually differ. The gasket of a top seal closure is frequently what is referred to as a "flowed-in" plastisol gasket composition which is poured into a channel around the periphery of the (inverted) insert disk and sets up in place without molding. In contrast, a special, more expensive composition having low oxygen permeability is generally used for side seal closures. Moreover, this material is usually molded under applied pressure to the desired configuration, and cures relatively slowly. These factors all contribute to a slower production rate and higher cost of closures for meat products, in comparison to closures for non-meat products.
Recently the baby food packing industry has started demanding closures which are tamper indicating (as by visible separation of a band around the closure upon initial opening) as well as pressure venting. This further complicates the manufacture of suitable packages. Moreover, food distributors require the packages for their meat products to have the same outward appearance as those for non-meat food products, so as not to diffuse the packaging "image" of the particular distributor. However, the cost of the special construction required to provide a side seal for meat products is significantly higher than that required for a similar package with just a top seal, so it is quite expensive to make all the closures look like the meat closures, which are used on only a small proportion of the total.